Researches at the University of Tokyo have developed what they are calling organic flash memory, which shares its basic structure with flash memory but is made entirely of organic materials. Its erasing and reading voltages are rated at a low 6V and 1V, respectively, though data can only be written and erased more than 1,000 times. Foreseen uses of the new memory include large area sensors, electronic paper and other big electronic devices, though the founders admit the technology's memory retention time of only one day needs to be extended.

Developed by a group headed up by Takeo Someya and Tsuyoshi Sekitani, professor and research associate at the Department of Electrical Engineering and Information Systems, Graduate School of Engineering, the University of Tokyo, the prototype uses a polyethylene naphthalate (PEN) resin sheet as a substrate with an array of 26 by 26 2T memory cells placed on it. The sheet is flexible enough to be bent until its radius is 6mm without causing damage. The short memory retention can be increased by reducing the size of the element and employing a longer self-assembled monolayer (SAM), which is made of phosphoric acid with an alkyl chain. In the prototype, it is 2nm thin. [via Tech-On]

The memory array. The white parts are control gates, and the watery blue parts under the three electrodes are pentacene.

The structure of the intelligent pressure sensor. From the bottom, they are an organic memory sheet, a pressure-sensitive rubber sheet and an electrode sheet.

The University of Tokyo developed the "organic flash memory," a non-volatile memory that has the same basic structure as a flash memory and is made with organic materials.

The erasing and reading voltages of the new flash memory are as low as 6V and 1V, respectively. Data can be written in and erased from the memory more than 1,000 times.

With its flexibility, the flash memory can be used for large-area sensors, electronic paper and other large-area electronic devices if its memory retention time can be extended, the university said.

The organic flash memory was developed by a research group led by Takeo Someya and Tsuyoshi Sekitani, professor and research associate at the Department of Electrical Engineering and Information Systems, Graduate School of Engineering, the University of Tokyo. The research results were published on the 11th December 2009 issue of Science magazine.

The research group used a polyethylene naphthalate (PEN) resin sheet as a substrate and arranged 26 x 26 2T memory cells in an array structure on it. The sheet is flexible, and the university confirmed that it can be bent until its curvature radius reaches 6mm without causing mechanical or electrical degradation. The university also confirmed that the sheet can be used as a non-volatile memory.

Furthermore, the research group made an "intelligent pressure sensor," which can retain a pressure image in the sheet, by integrating the memory array and a pressure sensor.

The new memory is called "organic flash memory" because it is equipped with floating-gate transistors, which are also used for silicon-based flash memories. Specifically, the PEN substrate is mounted with aluminum (Al) control gate electrodes, insulating films, aluminum floating gates, organic semiconductor pentacene, and source and gate electrodes made of gold (Au).

The insulating film was made by using a self-assembled monolayer (SAM), which is made of a kind of phosphoric acid having an alkyl chain (CH2-CH2-CH2-...), in addition to AlOx. The SAM is as thin as 2nm.

There are non-volatile memories developed in the past. One is made by using ferroelectric materials, and the other has a floating-gate structure like the latest non-volatile memory. However, it is difficult to lower the writing and erasing voltages of the former memory than 20V. The latter memory also has a high erasing voltage. And it becomes unstable when exposed to air because its memory properties fluctuate due to the nonconstant thickness of the insulating film.

This time, the university researchers employed an SAM whose insulating film does not require thickness control to reduce the variation in memory properties. Moreover, this SAM is stable in the atmosphere.

As a disadvantage, the new organic flash memory has a memory retention time of only one day. But this can be drastically improved by reducing the size of the element and employing an SAM with a long molecular length, the university said.

Japanese government selected the "dependable wireless solid-state drive" as a new research theme of the Core Research for Evolutional Science and Technology (CREST), a project led by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), for fiscal 2009.

The goal of the research is to realize an SSD that is highly reliable in terms of data retention time and the number of rewritable times and stable against various error factors such as loose connection, power-off while in use, water exposure and electrostatic discharge (ESD) caused by contact with human body.

To meet that goal, MEXT aims to develop a "wireless SSD" capable of wireless communication (10Gbps) and power transmission at a distance of 1mm and a host system for it. Also, the ministry plans to equip the SSD with a NAND flash memory of several terabytes.

In the CREST projects, which are allocated a large budget, researchers work together in three to five years. This time, the research is led by Ken Takeuchi, associate professor at the Department of Electrical Engineering and Information Systems, Graduate School of Engineering of the University of Tokyo, Tadahiro Kuroda and Hiroki Ishiguro, electrical engineering professor and associate professor, respectively, at Keio University.

Takeuchi, Kuroda and Ishiguro will take charge of developing an error-correcting system (for wireless communication and memory), wireless communication technology and power transmission and analogy technologies, respectively. The research will be conducted in collaboration with major consumer electronics manufacturers and memory manufacturers.

"I hope that Japanese universities, too, will create a device based on a new concept with practical use in mind," Takeuchi said.